This invention relates to hydromechanical transmissions. More specifically, this invention relates to a hydromechanical transmission with a drive line arrangement that accommodates the need for increasing vehicle speeds.
There are a number of skid steer vehicles that require transmissions having the capability to provide a separate controlled speed output to each side of the vehicle in order to steer the vehicle. These include skid steer loaders, crawler tractors and loaders, tracked farm tractors, asphalt pavers and utility machines. These vehicles may have wheels or tracks, and if wheeled, may have either a fixed or variable wheel geometry. In order to steer many of these vehicles have a hydrostatic transmission for each side of the vehicle with a separate speed control for each transmission, typically referred to as a dual path transmission. These dual path transmissions are usually operated with a control input for each transmission in order to achieve both steering and forward or reverse motion control.
Transmission output drives for skid steer vehicles are typically located adjacent to the wheels or the tracks and are closely coupled to the wheel or track drive. This might include gears or chains depending on the vehicle needs. The engine may be positioned longitudinally with respect to the vehicle direction of travel, or transversely in order to accommodate space or weight distribution needs. The maximum output speed may vary according to the vehicle vocation. There are typically auxiliary hydraulic power requirements to supply work function implements. Prior drive line technology for skid steer vehicles includes dual path hydromechanical transmissions. These drive lines typically include portions of the two transmissions integrated in the same housing. Many of these prior art drive lines have drive shafts with angularly flexible connections (U-joints) at the input or output of the hydromechanical transmission. Many dual path hydromechanical transmissions have portions directly attached to the engine.
Hydromechanical transmissions are characterized by a hydrostatic transmission power path in parallel with a mechanical power transmission path, arranged in a manner to decrease the average power flow through the hydrostatic portion and thereby increase operating efficiency. Typically, the mechanical power path includes a planetary gear set which acts to sum the power flows at either the input or output end of the transmission.
The existence of parallel power paths create the possibility of reducing the output speed range or torque ratio in order to further reduce transmitted hydrostatic power. This then requires multiple ranges or “modes” to achieve the full torque and speed range of the transmission. The impact of multiple modes is to improve efficiency and sometimes to reduce cost. Multiple mode hydromechanical transmissions are usually accomplished by reusing the hydrostatic components and clutching to a different mechanical component. Usually the modes are arranged so that there is no ratio change during the mode change in order to have continuous speed or torque delivery.
There is a need in the art to provide greater output speeds in the forward, direction of travel to accommodate the desire to travel faster. Additionally, high efficiency of operation is also required in the art to minimize operating costs. Other needs in the art include providing a compact size of vehicle having transmissions that are easy to install and overall have a reduced cost. Specifically, the vehicles are work vehicles with high maneuverability requirements including counter rotation and spin turns. Many of the vehicles have the requirement for continuous forward and reverse cycling. Additionally desirable is to have a continuous ratio throughout the vehicle's speed range in order to allow maximum flexibility for the driver or the work to be done.
Thus, a principle object of the present invention is to provide a vehicle using a hydromechanical transmission that allows for increasing vehicle speeds.
Yet another object of the present invention is to provide a vehicle operating system that has increased efficiency with reduced operating cost.
Another object of the present invention is to provide an operating system for a vehicle that is compact in size.
These and other objects, features or advantages of the present invention will become apparent from the specification and claims.